Disk diffusers are commonly used in a variety of gas diffusion applications, including the aeration and/or mixing of water and wastewater. Disk diffusers often use membranes having perforations which open when pressurized gas is applied to the system. Although membrane disk diffusers have been successful in large part, they have not been wholly without problems.
In particular, achieving an effective seal between the periphery of the membrane and the diffuser body has been difficult. One technique is to extend the membrane over the edge of the diffuser body and apply a clamping band around the diffuser body to secure the membrane and maintain a seal. This technique requires a stainless steel clamping band which is expensive and is also difficult to install and remove. At times, the membranes are blown off of the diffuser in this type of device.
Sealing small units is relatively easy because there is less surface to seal and less force required to create a reliable seal. For smaller units, the industry has moved in recent years toward the use of a threaded retainer ring to clamp the membrane in place. The membrane is provided with an integral circumferential sealing bead which acts as an O-ring type seal as well as an anchor that holds the membrane on the diffuser body when the retainer ring is tightened.
The effectiveness of the seal depends upon tightening the retainer ring enough that the integral sealing bead is fully compressed against the diffuser body. With larger diffusers, the force required to tighten the retainer ring sufficiently to prevent air leakage is substantial. Compounding this problem is the fact that the durometer and other characteristics of the membrane are selected for maximum oxygen transfer and aeration performance. Because the sealing bead is formed as part of the membrane, it necessarily has the same durometer and other physical characteristics as the membrane. This can compromise the ability of the sealing bead to effect a good seal. For example, the membrane durometer is typically relatively high for good oxygen transfer, but the hardness has the undesirable effect of requiring an excessively large force to compress the sealing bead.
There are other problems that can make the tightening force unduly high. The diffuser body may be warped or otherwise misshapened such that adequate contact between the membrane sealing bead and the diffuser body at all points is difficult to achieve. The diameter and shape of the sealing bead may be out of tolerance in some cases, or the membrane may be uneven or have flash from the manufacturing process that inhibits its ability to provide a uniform seal. Additionally, threads on the retainer ring may be imperfect or out of tolerance, thus preventing proper tightening of the retainer ring against the sealing bead.
Large disk diffusers are generally desirable for reasons of efficiency and economy. However, their size can result in extremely large force requirements for adequately compressing the membrane bead to create a gas tight seal. The added structural capacity that is necessary for the diffuser to withstand the large forces that need to be applied can lead to excessive costs. Mechanical leverage tools are often necessary, and they add to the costs that are involved and can result in over-tightening that creates a variety of adverse effects.
During operation in a wastewater treatment application or in many other applications, organic material, sand, grit and other substances can become lodged in the threads. Combined with the extreme tightening force that must be applied during installation, these materials can make it difficult to disassemble the diffusers for maintenance, repair, or replacement of the membranes. Not uncommonly, retainer rings or diffuser bodies are bent, broken, or otherwise damaged during disassembly efforts.